A geological fault is a fracture within the Earth’s crust where two blocks of rock have moved relative to one another. Movement along these fractures can be rapid, causing an earthquake, or occur slowly over long periods. Geologists classify faults based on the direction of this relative displacement. Normal faults are one of the three primary types, distinguished by their characteristic movement. Understanding their cause requires looking at the immense forces generated by the slow motion of tectonic plates, which determines the type of stress applied to the rock.
Defining the Normal Fault Structure
Every inclined fault is defined by two rock masses: the footwall and the hanging wall. The footwall is the block that lies beneath the fault plane, while the hanging wall is the block situated above it.
In a normal fault, the movement is a specific type of vertical displacement called dip-slip motion. The defining characteristic is that the hanging wall block moves downward relative to the footwall block. This downward slip causes an extension or lengthening of the crust.
The fault plane is often steeply inclined, commonly dipping between 45 and 90 degrees. Displacement along a single normal fault can range from less than a meter to thousands of meters over geologic time. This relative drop distinguishes a normal fault from a reverse fault, where the hanging wall moves upward.
The Primary Geological Force
The force responsible for creating normal faults is tensional stress, also known as extensional stress. Tensional stress acts to stretch or pull a rock body apart. When this force exceeds the rock’s strength, the rock fractures in a brittle manner, resulting in a fault.
This pulling-apart motion causes the Earth’s crust to lengthen and thin out horizontally. The resultant strain manifests as the downward slip of the hanging wall block. The maximum principal stress in this environment is vertical, with the least stress directed horizontally, which facilitates the vertical drop.
This mechanism contrasts with the other two main fault types. Reverse faults are caused by compressional stress, which pushes rock blocks together, forcing the hanging wall up. Strike-slip faults are caused by shear stress, where rock blocks slide horizontally past each other.
Geological Settings of Normal Faults
Tensional stress, the driving force for normal fault formation, is most prevalent in specific, large-scale tectonic environments. The most common setting is at a divergent plate boundary, where two tectonic plates are actively moving away from each other. As the plates separate, the crust is stretched and thinned, which provides the necessary extensional stress to create numerous normal faults.
This process is visible along the mid-ocean ridge system, which is the world’s longest continuous geological feature. Here, the ocean floor is constantly pulling apart, and normal faults bound narrow, down-dropped blocks along the ridge axis.
On continents, tensional forces create continental rift zones, areas where a continent is beginning to tear apart. A prime example is the East African Rift System, a vast network of normal faults where the crust is being stretched over a distance of thousands of kilometers. Another well-known region is the Basin and Range Province in the western United States.
Landforms Created by Normal Faulting
The cumulative effect of widespread normal faulting creates a distinctive landscape known as horst and graben topography. These terms describe the alternating raised and lowered blocks of crust formed between parallel normal faults under extensional stress.
A graben is the down-dropped block of crust that sinks between two parallel normal faults whose dip angles face toward each other. These features often form valleys or basins, and a large-scale graben is commonly referred to as a rift valley. The Jordan-Dead Sea depression and Death Valley are prominent examples.
Conversely, a horst is the uplifted block of crust situated between two normal faults whose dip angles face away from each other. These raised blocks typically form mountain ranges, plateaus, or ridges, such as the Vosges Mountains in France.
These fault-bounded depressions and uplifts have environmental and economic consequences. Grabens tend to collect sediment over time, sometimes forming reservoirs for oil and gas. Furthermore, the deep fractures associated with normal faults can allow for geothermal activity, as heat from the Earth’s interior rises closer to the surface.